In preparation for future trials of biomedical and behavioral interventions, it is critical to establish HIV incidence in high risk cohorts who have benefited from the best, most affordable prevention programs available to them locally. Three-quarters of African adults are in cohabiting unions, where most new infections occur. One Love campaigns promote monogamy, but unless both partners in a sexual dyad - whether married or concurrent - have the same infection status, being faithful is not protective. In this application, Couples' Voluntary HIV Counseling and Testing (CVCT) provided to cohabiting couples in Zambian government clinics will be extended to concurrent partners, and the impact of joint testing on incidence of HIV acquired within and outside the union will be measured. Discordant couples are critical to HIV prevention research: They allow study of virologic, immunologic and immunogenetic correlates of 'contagion' and 'vulnerability' in male-to-female and female-to-male transmission, and testing of biomedical and behavioral interventions in both HIV+ and HIV- partners. Comparison of transmitted viruses in donor and recipient clarify the selection process that a vaccine must overcome. Comparison of HIV incidence in several East-Central African observational discordant couple cohorts with (3.5/100 PY) and without (10.2/100 PY) CVCT yields an estimated reduction of two-thirds. In counseled discordant couples in Lusaka, Zambia, HIV incidence between spouses is 6/100 PY, and an additional 1.5/100 PY are infected by concurrent partners. Only one small published study of uncounseled Zambian discordant couples is available and reported an incidence of 21/100 PY, thus there is less consensus about the effect size of CVCT in southern Africa. Aim 1 will examine the incidence and origin of infections acquired prior to CVCT using viral genome testing and will quantify the prevention impact of CVCT in discordant couples. UNAIDS and DHS data indicate that most extramarital contacts are with longstanding steady partners. The potential impact of joint testing with concurrent partners (CPT) on HIV infections acquired outside marriage in both discordant and concordant negative couples has not been explored. A formative evaluation phase will precede development (Aim 2) of a CPT intervention adapted from CVCT procedures developed under RO1 MH 66767. Forty clinics with CVCT services managed by the research team will be randomized to provide CPT (Aim 3). The incidence of new infections acquired outside marriage will be compared in the CVCT only vs. CVCT+CPT clinics. Despite counseling and free services, only 10% of discordant couples with HIV- male partners request male circumcision (MC). Like CVCT, MC is a locally affordable and proven prevention strategy which must be offered to participants in future trials. Ancillary Aim 4 will investigate obstacles to male circumcision (MC) to optimize adherence with ethical standards in future trials.